1. Field of the Invention
The present invention relates to surface acoustic wave devices for use in electronic circuits of mobile communications devices and the like. More particularly, the invention relates to surface acoustic wave devices comprising surface acoustic wave resonators.
2. Description of the Related Art
High-frequency devices utilizing surface acoustic waves (SAWs) have found wide use in filters, oscillators and similar electronic circuits for use in mobile communications devices such as portable telephones.
SAW devices in use include those of the resonator type which comprise as formed on a piezoelectric substrate a SAW resonator or a plurality of SAW resonators connected together, the SAW resonator comprising an interdigital transducer (hereinafter referred to briefly as an "IDT") and a grating reflector (hereinafter referred to merely as a "reflector").
FIG. 1A shows the basic construction of SAW resonators which comprises an IDT 1 composed of many pairs of electrode digits 10a, 10b arranged on a piezoelectric substrate 3, and reflectors 2, 2 arranged respectively on opposite sides of the IDT 1.
When a high-frequency voltage is applied across the electrode digits 10a, 10b of the IDT 1, surface acoustic waves are produced in the vicinity of the substrate surface as a result of a piezoelectric effect. The surface acoustic waves are excited most efficiently to propagate on the substrate 3 when the wavelength matches the electrode digit pitch Li of the IDT 1 since the electrodes then excite surface acoustic waves of the same phase.
With SAW devices, therefore, the digit pitch Li is termed an "electrode period."
With reference to FIG. 1A and FIG. 1B, suppose the electrode period of the IDT 1 is Li, the width of electrode digits 10a, 10b of the IDT 1 is Di, the interdigitation length of the electrode digits 10a, 10b is Wi, and the film thickness of the electrodes is Hi.
The reflector 2 embodying the present invention comprises an array of metal strip electrodes 20 formed on a piezoelectric substrate 3 by the same procedure as the IDT 1. Examples of electrode constructions of reflectors 2 include an open type 21 wherein strip electrodes 20 are not connected at their opposite ends as shown in FIG. 2, a short-circuit type 22 wherein strip electrodes 20 are connected together at their opposite ends as shown in FIG. 3, and an interdigital type 23 having the same electrode construction as the IDT 1 as seen in FIG. 4.
The spacing between the electrodes in the reflector 2 is approximately equal to the spacing between the electrode digits in the IDT 1. Accordingly, the length Lg in the reflector 2 corresponding to the electrode period Li of the IDT 1 as shown in FIG. 1B is termed the electrode period of the reflector 2.
With reference to FIG. 1B to FIG. 4, the electrode period of the reflector 2 is Lg, the width of the electrodes is Dg, the interdigitation length of the electrodes is Wg, and the film thickness of the electrodes is Hg.
With the development of mobile communications devices of higher performance and reduced size in recent years, there is a greater need for SAW devices of improved performance in smaller sizes. In reducing the size of SAW devices of the resonator type, it is essential to miniaturize the SAW resonator itself as a component of the device.
The SAW resonator can be miniaturize, for example, by:
(1) using a substrate of low acoustic velocity to give a reduced SAW propagation velocity and thereby shortening the IDT period Li and reflector period Lg, PA1 (2) using a substrate which has a large electromechanical coupling factor indicating the ability to convert electric signals to surface acoustic waves and thereby decreasing the number and interdigitation lengths Wi, Wg of electrodes, PA1 (3) using an electrode material of higher excitation efficiency and improved reflection efficiency and thereby reducing the number and interdigitation lengths Wi, Wg of electrodes, and PA1 (4) using an IDT structure of higher excitation efficiency or a reflector structure of high reflection efficiency and thereby reducing the number and interdigitation lengths Wi, Wg of electrodes.
The measures (1) to (3) require the use of a different substrate material or electrode material, necessitating a great change in the electrode structure or production process, and are therefore not feasible, whereas the resonator can be miniaturized by the measure (4) without greatly altering the conventional method of design or the production process since there is no need to change the material.
Of the parameters which are thought to be relevant to the performance of SAW devices, those associated with the electrodes include the electrode occupancy ratios (the ratio of the electrode area in the region of electrode interdigitation) 2.times.Di/Li, 2.times.Dg/Lg, the ratio of the IDT period Li to the reflector period Lg, Li/Lg, and the ratios of the electrode thicknesses Hi and Hg of the IDT and the reflector to the SAW wavelength .lambda., Hi/.lambda. and Hg/.lambda.. Accordingly, the inventor conducted experiments on reflectors 2 of the foregoing three different electrode structures (FIGS. 2 to 4) to check the reflectors 2 for SAW reflection efficiency with these parameters altered, and found that the parameters giving an improved reflection efficiency differ from structure to structure, and that especially in the case where the reflector 2 is of the open type 21 (FIG. 2) or of the interdigital type 23 (FIG. 4), the optimum electrode occupancy ratio 2.times.Dg/Lg is greatly different from the value (about 0.5) conventionally thought optimum.
An object of the present invention is to provide a reflector having a high reflection efficiency for use in SAW devices which comprise as formed on a piezoelectric substrate one or a plurality of resonators connected together, each of the resonators comprising an IDT and a reflector.